Error correcting typewriters are well known. In some of these a single composite two color ribbon is used; see, for example, my prior U.S. Pat. No. 3,154,183, issued Oct. 27, 1964, for "Ribbon Shift For Error-Obliterating Typewriters" and No. 3,595,362, issued July 27, 1971, for "Typewriter Backspace and Ribbon-Field Control." In other prior art, two separate ribbons, one a writing ribbon and one a correcting ribbon, are used; see, for example, my prior U.S. Pat. No. 3,149,711, issued Sept. 22, 1964, for "Error-Correcting Typewriter" and U.S. Pat. No. 3,204,746, issued Sept. 7, 1965, for "Typewriter With Error-Correction Features."
The prior art also shows that a so-called correcting key may be added to a typewriter for backspacing to the point of the error and selecting the correcting ribbon field, see my U.S. Pat. Nos. 3,149,711 and 3,595,362, Ozimek et al. U.S. Pat. No. 3,729,081 and Davidge et al. U.S. Pat. No. 3,799,316.
It is also taught in some of my prior copending applications that the error correcting key may be carried by an attachment that may be added to an existing typewriter. In this respect see my prior copending applications Ser. No. 470.476, filed May 16, 1974, now U.S. Pat. No. 3,927,748, issued Dec. 23, 1975, for "Attachment for Typewriters" and Ser. No. 496,602, filed Aug. 12, 1974, now U.S. Pat. No. 3,927,749, issued Dec. 23, 1975, for "Apparatus for Supplementing the Operation of the Ribbon Selecting and Backspacing Means of a Typewriter."